The present invention is related in general to the field of semiconductor devices and processes and more specifically to the process of wire bonding to bond pads of copper-metallized integrated circuits.
In integrated circuits (IC) technology, pure or doped aluminum has been the metallization of choice for interconnection and bond pads for more than four decades. Main advantages of aluminum include easy of deposition and patterning. Further, the technology of bonding wires made of gold, copper, or aluminum to the aluminum bond pads has been developed to a high level of automation, miniaturization, and reliability. Examples of the high technical standard of wire bonding to aluminum can be found in U.S. Pat. No. 5,455,195, issued on Oct. 3, 1995 (Ramsey et al., xe2x80x9cMethod for Obtaining Metallurgical Stability in Integrated Circuit Conductive Bondsxe2x80x9d); U.S. Pat. No. 5,244,140, issued on Sep. 14, 1993 (Ramsey et al., xe2x80x9cUltrasonic Bonding Process Beyond 125 kHzxe2x80x9d); U.S. Pat. No. 5,201,454, issued on Apr. 13, 1993 (Alfaro et al., xe2x80x9cProcess for Enhanced Intermetallic Growth in IC Interconnectionsxe2x80x9d); and U.S. Pat. No. 5,023,697, issued on Jun. 11, 1991 (Tsumura, xe2x80x9cSemiconductor Device with Copper Wire Ball Bondingxe2x80x9d).
In the continuing trend to miniaturize the ICs, the RC time constant of the interconnection between active circuit elements increasingly dominates the achievable IC speed-power product. Consequently, the relatively high resistivity of the interconnecting aluminum now appears inferior to the lower resistivity of metals such as copper. Further, the pronounced sensitivity of aluminum to electromigration is becoming a serious obstacle. Consequently, there is now a strong drive in the semiconductor industry to employ copper as the preferred interconnecting metal, based on its higher electrical conductivity and lower electromigration sensitivity. From the standpoint of the mature aluminum interconnection technology, however, this shift to copper is a significant technological challenge.
Copper has to be shielded from diffusing into the silicon base material of the ICs in order to protect the circuits from the carrier lifetime killing characteristic of copper atoms positioned in the silicon lattice. For bond pads made of copper, the formation of thin copper(I)oxide films during the manufacturing process flow has to be prevented, since these films severely inhibit reliable attachment of bonding wires, especially for conventional gold-wire ball bonding. In contrast to aluminum oxide films overlying metallic aluminum, copper oxide films overlying metallic copper cannot easily be broken by a combination of thermocompression and ultrasonic energy applied in the bonding process. As further difficulty, bare copper bond pads are susceptible to corrosion.
In order to overcome these problems, a process has been disclosed to cap the clean copper bond pad with a layer of aluminum and thus re-construct the traditional situation of an aluminum pad to be bonded by conventional gold-wire ball bonding. A suitable bonding process is described in U.S. Pat. No. 5,785,236, issued on Jul. 28, 1998 (Cheung et al., xe2x80x9cAdvanced Copper Interconnect System that is Compatible with Existing IC Wire Bonding Technologyxe2x80x9d). The described approach, however, has several shortcomings.
First, the fabrication cost of the aluminum cap is higher than desired, since the process requires additional steps for depositing metal, patterning, etching, and cleaning. Second, the cap must be thick enough to prevent copper from diffusing through the cap metal and possibly poisoning the IC transistors. Third, the aluminum used for the cap is soft and thus gets severely damaged by the markings of the multiprobe contacts in electrical testing. This damage, in turn, becomes so dominant in the ever decreasing size of the bond pads that the subsequent ball bond attachment is no longer reliable.
A low-cost structure and method for capping the copper bond pads of copper-metallized ICs has been disclosed on U.S. patent application Ser. No. 60/183,405, filed on 18 Feb. 2000. The present invention is related to that application. An urgent need has arisen for a reliable method of bonding wires to capped bond pads which combines minimum fabrication cost with maximum up-diffusion control of metals potentially capable of impeding subsequent wire bonding. The bonding method should be flexible enough to be applied for different IC product families and a wide spectrum of design and process variations. Preferably, these innovations should be accomplished while shortening production cycle time and increasing throughput, and without the need of expensive additional manufacturing equipment.
The present invention discloses a robust, reliable and low-cost metal structure and process enabling electrical wire connections to the interconnecting copper metallization of integrated circuits (IC). The structure comprises a layer of barrier metal that resists copper diffusion, deposited on the non-oxidized copper surface in a thickness such that the barrier layer reduces the diffusion of copper at 250xc2x0 C. by more than 80% compared with the absence of the barrier metal. The structure further comprises an outermost layer reduces the diffusion of the barrier metal at 250xc2x0 C. by more than 80% compared with the absence of the bondable metal. Finally, a metal wire is bonded to the outermost layer for metallurgical connection.
The barrier metal is selected from a group consisting of nickel, cobalt, chromium, molybdenum, titanium, tungsten, and alloys thereof. The outermost metal layer is selected from a group consisting of gold, platinum, and silver.
The present invention is related to high density and high speed ICs with copper interconnecting metallization, especially those having high numbers of metallized inputs/outputs, or xe2x80x9cbond padsxe2x80x9d. These circuits can be found in many device families such as processors, digital and analog devices, logic devices, high frequency and high power devices, and in both large and small area chip categories.
It is an aspect of the present invention to be applicable to bond pad area reduction and thus supports the shrinking of IC chips. Consequently, the invention helps to alleviate the space constraint of continually shrinking applications such as cellular communication, pagers, hard disk drives, laptop computers and medical instrumentation.
Another aspect of the invention is to fabricate the bond pad metal caps by the self-defining process of electroless deposition, thus avoiding costly photolithographic and alignment techniques.
Another aspect of the invention is to be guided by the metal diffusion coefficients for selecting the appropriate pair of metals and the coordinated layer thicknesses in order to minimize up-diffusion at the elevated bonding temperatures and subsequent bond-inhibiting chemical reactions.
Another aspect of the invention is to advance the process and reliability of wafer-level multi-probing by eliminating probe marks and subsequent bonding difficulties.
Another object of the invention is to provide design and process concepts which are flexible so that they can be applied to many families of semiconductor products, and are general so that they can be applied to several generations of products.
Another object of the invention is to use only designs and processes most commonly employed and accepted in the fabrication of IC devices, thus avoiding the cost of new capital investment and using the installed fabrication equipment base.
These objects have been achieved by the teachings of the invention concerning selection criteria and process flows suitable for mass production. The non-oxidized surface of the copper of the bond pad is seeded by a metal such as palladium and covered with a layer of a barrier metal such as nickel. The thickness of this barrier layer has to be such that it prevents excessive copper up-diffusion at the elevated temperatures of the bonding operation. The outermost layer is a bondable metal such as palladium or gold. The layer thickness has to be such that it prevents nickel up-diffusion to the surface, where it would oxidize and impede wire bonding. In mass production, the various metal layers are deposited by electroless plating, thus avoiding the need for expensive photolithographic definition steps.
The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.